In the manufacture of television receivers, a number of different adjustments are made to the receivers prior to shipping them from the factory to insure the best possible operation of the receiver once it is placed in the customer's home. For color television receivers, one of the adjustments which must be made is an adjustment of color purity. Each television receiver has purity adjustment magnets or rings on the neck of the cathode ray tube to cause the electron beams to strike the correct phosphor dots for the desired colors, thereby causing the "purity" of the colors reproduced on the cathode ray tube screen to be as close as possible to the ideal or perfect color reproduction which the receiver is capable of achieving.
Generally, one or the other of two methods of adjusting color purity of a television receiver during its manufacture or at a service center is used. One of these is the "microscope" method, and the other is known as the "red-ball" method. Both of these adjustments are made after the receiver has reached its normal operating temperature, so that the temperature of the shadow mask on the picture tube has been stabilized.
The microscope method is used to observe directly the landing positions of the electron beams of the cathode ray tube relative to the phosphor dot triads. This is accomplished by observing the triads in a microscope and adjusting the purity magnets, so that the three electron beams all land correctly on their appropriate phosphor dots at the center of the screen. Ideally, the beams land on the geometric centers of the phosphor dots, and this can be observed through the microscope. Relatively complex apparatus, such as a periscope or the like, is necessary in addition to the microscope in making a purity adjustment by this method. It also is relatively time consuming and restrains the operator making the adjustment to the physical location of the microscope.
The red-ball method of making a purity adjustment is accomplished with only the red gun of the cathode ray tube operating. The deflection coil assembly is pulled back on the neck of the cathode ray tube as far as it will go to place a red ball on the center of the cathode ray tube screen. The purity magnets are adjusted so that the center of the red ball is at the center of the cathode ray tube screen. If the cathode ray tube has a center beam landing precompression feature, the red ball is adjusted to be located slightly offset from the screen center in the direction of the red gun of the tube. Once the purity magnets have been adjusted to accomplish this location of the red ball, the yoke assembly is relocated to its correct axial position and final adjustments are made to give a pure red raster over the entire screen. The determination of the proper location of the red ball for the purity adjustment and the determination of the purity of color of the red raster over the entire screen are subjective determinations made by the operator effecting the adjustments.
The red-ball purity adjustment method is a popular one because of its simplicity. This method, however, only results in an approximate purity setting, since the purity adjustment is effected with the deflection yoke pulled back from its normal operating position. The error introduced by making the purity adjustment in this manner generally is small enough that it does not noticeably affect the operation of the cathode ray tube. It is possible, however, to make a purity adjustment by this method which includes substantial error significant enough to be readily observable when the television receiver is subsequently operated.
Another method which has been developed for making a color purity adjustment is a modification of the red-ball method. This modified method operates with the deflection yoke assembly of the cathode ray tube in its normal operating position. The red gun then is placed in operation, the green and blue guns are turned off. A circular coil then is placed in front of the screen on which the unmodulated red raster is displayed. A direct current is passed through the coil and the resulting field rotates the beam landing around the center of the coil to cause a clearly defined red ball to appear at the approximate center of the coil. The actual location of the ball depends upon the adjustment of the color purity magnets which then can be adjusted until the red ball appears in the center of the auxiliary coil. To aid the operator in making a purity adjustment with this method, a template may be positioned inside the coil to indicate the size and location of the red ball for a properly adjusted receiver. In this system, the final yoke adjustment for establishing the purity at the sides of the cathode ray tube, however, is done in the same manner as with the red-ball method previously described. A subjective determination of the location of the red ball and of the overall color purity when the yoke adjustment is made still occurs in the practice of this method.
It is desirable to provide an apparatus and method for adjusting the purity of the cathode ray tube of a color television receiver which removes, to as great an extent as possible, the subjective determination of the operator making the adjustment. It also is desirable to effect purity adjustments of a color cathode ray tube quickly and simply with a minimum chance for operator error and with a minimum amount of subjective determination on the part of the operator making the adjustments.